Agricultural machines, such as forage harvesters for example may be equipped with implements to be worked over the ground. The implement may carry out processes such as cutting grass, grain, maize or working the soil. Owing to the variety of working processes and terrain conditions a self propelled machine must be capable of carrying and moving a wide range of different implements of varying weights under different working conditions.
For some processes such as heavy soil tillage, the full weight of the implement rests on the ground. In other working conditions the full weight of the implement will be carried by the machine so that the implement does not touch the ground. This would be the case for example with maize harvesting. In other cases, such as grass harvesting, part of the weight of the implement is carried by the machine and part of the weight is borne by the ground.
Typical lifting arrangements on agricultural machines comprise one or more carrying arms to which an implement is attached and which can be moved to lift and lower an implement. A carrying arm is movable around one, or more pivot points by one or more hydraulic cylinders. There is therefore mechanical and hydraulic friction with such an arrangement. The amount of friction will depend on the direction of movement of the arms and will be different from the extending and retracting stroke of the cylinders which will lead to hysteresis.
Some implements which are attached to the machine are to be moved in one direction only (upwards) since to move the implement downwards using the cylinders could damage the ground or implement. Such implements require a single acting hydraulic cylinder or a double acting hydraulic cylinder that can be used in single acting mode. Typically, the hydraulic arrangement is arranged such that pressurisation of the cylinder extends the piston rod which raises a carrying arm and implement. When the cylinder is not pressurised the piston retracts slowly under the weight of the attached implement lowering the carrying arm and implement to the ground.
Owing to mechanical and hydraulic friction in a hydraulic arrangement, the weight of the attached implement may not be sufficient to contract the cylinder to lower the carrying arm. Such implements will require a double acting hydraulic cylinder so that the implement can be both lifted and lowered to the ground by controlling the pressure exerted on the hydraulic cylinders.
Also, for maintenance of a forage harvester it is necessary to dismount an intake from the carrying arms to allow maintenance work to the machine to be carried out. This means that there is no weight attached to the carrying arms and therefore if a single acting hydraulic cylinder is used the cylinders cannot be retracted and therefore the carrying arms cannot be lowered unless an external force is applied to the carrying arms.
It is a known problem to retract a double acting cylinder which is connected to a single acting valve arrangement as it must be pushed in manually. This means the attached carrying arm must be pushed downwards manually if it is to be lowered.
Alternatively, if the carrying arm has already been detached for maintenance purposes, the attachment rods (to which the carrying arms are mounted) must pushed in manually. It is known that hydraulic cylinders used on medium or large size machines cannot be pushed in manually because the frictional force is so high that it can not be overcome by human force alone.
It is an aim of the invention to overcome the problem set out above and provide a hydraulic arrangement for moving a carrying arm on an agricultural machine in which the function of the hydraulic cylinders can be easily switched between a single acting mode and a double acting mode using a single valve arrangement.
It is a further aim of the invention to provide an hydraulic arrangement which is suitable for moving both heavy and light implements attached to a carrying arm in a manner that the pressure required to lift a light implement will be raised artificially to the same level that is required to lift a heavy implement.
In accordance with the invention there is provided an hydraulic arrangement for lifting and/or lowering at least one implement carrying arm on an agricultural machine having the features of claim 1.
In case of maintenance and service it may be necessary for a person to position himself under an implement mounted to a forage harvester. In order to prevent serious injuries due to an operator lifting or lowering the implement, there is legislation stating that it must be possible to lock and hold the at least one hydraulic cylinder so that its movement does not caused injury.
Preferably, the hydraulic arrangement comprises a blocking valve to prevent unwanted movement of the at least one cylinder.
Further preferred features of the invention are set out in the dependent claims.